1. Field of the Invention
The present invention relates to a toner suitable for use in copiers, electrostatic printing systems, printers, facsimiles, and electrophotographic systems such as electrostatic recording systems, and especially in a super-high-speed printing system which can be used in print-on-demand (POD) technology, as well as to a developer containing the toner.
2. Description of the Related Art
In recent years, market demands for energy saving and higher speed processing have increased for image forming apparatuses such as printers, copiers, and facsimiles. With the increase of such market demands, also in the field of electrophotographic toner (hereinafter, may be simply referred to as “toner”), a demand for a toner having excellent low-temperature fixability increases, while there is a need for a toner having conflicting properties to the low-temperature fixability such as offset resistance and heat resistant storage stability (blocking resistance). Particularly in toners produced through pulverization, a polyester resin advantageous in enhancing low-temperature fixability is increasingly used for a toner binder. However, particles produced using such polyester resin often has lower pulverizability than particles produced using a conventional styrene-acrylic resin, and thus such polyester resin has been disadvantageous in reduction of the particle size of the resultant toners, which reduction would have enhanced toner productivity and image quality.
Aiming to solve these problems, a toner containing a nonlinear cross-linked polyester resin, which is produced using a rosin for an acid component, has been reported as being a toner which has excellent low-temperature fixability and which can achieve both hot-offset resistance and heat resistant storage stability (see Japanese Patent Application Laid-Open (JP-A) No. 04-70765). Alternatively, there has been a toner containing a polyester resin which uses a purified rosin and an acid-modified rosin and, thereby, solves odor-related problems and improves heat resistant storage stability (see JP-A No. 2007-137910, JP-A No. 2007-292815, and JP-A No. 04-307557).
As a result of the incorporation of such rosin skeletons, the polyester resin used in these toners becomes hard and brittle, and excellent in pulverizability, however, becomes poor in mechanical durability. Consequently, some of the toner particles break by stirring stress within a developing apparatus and thereby lower toner's function, and, further, carriers and the inside of the developing apparatus are contaminated by the broken fine toner particles, resulting in significantly reduced developing performance.
On the other hand, in JP-A No. 2007-139811 and JP-A No. 2007-139812 toners which incorporate a purified rosin and bisphenol A structure for enhancement of the durability of the toners has been reported, however, these toners have not solved the above problems.
Meanwhile, in JP-A No. 08-54755, it is reported that electrostatic chargeability and development ability are improved by using a so-called capsule toner which is composed of core substances and an outer shell and which contains a colorant treated with a natural rosin or a rosin such as an abietic acid derivative in the core substances.
In addition, in JP-A No. 07-128911, it is reported that a color toner, incorporating a colorant surface-treated with abietic acid, becomes excellent in transfer efficiency, cleaning property, and charging stability, and able to achieve excellent full color images.
In these toners, rosin derivatives of heteroaromatic rings such as abietic acid derivatives of heteroaromatic rings are used, which in fact improves dispersibility of the colorants and achieves initial excellent transfer efficiency and charging stability. However, these rosin derivatives are low-molecular-weight substances and are not chemically bound to a binder resin, therefore, under friction caused by an image forming apparatus some of the toner particles that contain a significant amount of the low-molecular-weight rosin derivatives are likely to adhere to carriers in two-component developing systems and to a charge roller in one-component developing systems, leading to gradual loss of development ability.
In addition, when a fixing step is carried out by a contact heating system using a heating member such as heating roller, a high level of toner release property of a releasing agent from the heating member (hereinafter, may be referred to as “offset resistance”) is required. The offset resistance may be enhanced by disposing the releasing agent on the surface of a toner particle. For the enhancement of such offset resistance, the releasing agent may be contained in the toner particle before exuding on the surface of the toner particle, and thus the toner release property of the releasing agent is largely influenced by the state of the releasing agent in the toner particle. The releasing agent is preferably homogeneously dispersed in the toner particle in terms of stabilization of its quality. However, the dispersion diameter of the releasing agent in the toner particle cannot be reduced unlimitedly but preferably has a suitable value, because when the releasing agent has an excessively fine dispersion diameter it becomes difficult for the releasing agent to exude on the surface of the toner particle at the time of heating. A polyester resin produced using an aliphatic alcohol is highly compatible with general releasing agents, thus making less effective the toner release property of the releasing agents. The polyester resin produced by using an aliphatic alcohol is, however, very rigid due to its structure and can have a slightly high molecular weight at a fixed fixing temperature, and therefore tends to be superior in heat resistant storage stability to a styrene-acrylic resin. A styrene-acrylic resin, on the other hand, must have a low-molecular-weight to be able to melt at a low temperature, which makes the resin brittle and degrades the heat resistant storage stability of the resin. However, the styrene-acrylic resin is cheap and has large freedom of structural design such as in selecting a functional group, which freedom makes it easy to control the compatibility with the above-mentioned releasing agents.
Consequently, a variety of methods in which the styrene-acrylic resin and the polyester resin are mixed for use are proposed (JP-A No. 2003-255611; JP-A No. 2003-5432; JP-A No. 2002-365843). However, in these proposed toners the styrene-acrylic resin is less compatible with the polyester resin, causing separation between the two resins in the toner, thereby easily causing carrier contamination with resin components and alteration in physical properties.
In recent years, the market of the print-on-demand (POD) field has grown substantially, and printing market becomes in need of further sophisticated toners. The POD technology utilizing an electrophotographic printing method is well suited for printing a small number of copies and for variable printing and thus is greatly expected as an alternative to simple printing technology (“keiinsatu”). When the electrophotographic printing technology is used in the POD system, so as to operate under high linear speed conditions, the electrophotographic printing technology requires highly sophisticated low-temperature fixing technology.
In the meanwhile, since the POD systems are used in printing market, there is a need to achieve the electrophotographic process with substantially longer operating life than conventional electrophotographic systems for office use and domestic use. In particular, when fixing devices, which are members to be abraded most remarkably among members used in electrophotographic systems, have a short operating life, the outage of the printing machine itself is prolonged due to the replacement with a new fixing device, leading to degradation in printing capability. Thus, achieving longer operating life of POD systems is one of the important subjects to be addressed. The POD system is designed for achieving long operating life and therefore the toner consumption amount per POD system unit will be significantly large. Therefore, at present, the toners used in POD systems require to be more greatly improved, for decreasing abrasion rate of the fixing members, than toners used in conventional electrophotographic systems.